In a semiconductor integrated circuits, factors affecting the occurrence of electromigration, which may deform a conductor or cause the circuit to malfunction, are known to include element density, temperature, and crystal structure as explained in Takashi TOMIZAWA and Yasuo MATSUYAMA “Principle Of COMOSVLS1 Design—From the Viewpoint Of Systems,” Maruzen Co., Ltd., pp. 122, incorporated herein by reference. The adverse effects of temperature have not been sufficiently examined, so that specific constraints on temperature and the like are unknown.
For semiconductor integrated circuits used for automobiles or plant measurements or in other high-temperature environments (i.e., temperatures higher than room temperature), the effects of temperatures stress on the semiconductor integrated circuit must be sufficiently considered. In particular, with a sensor or similar device that amplifies a generated analog minor signal by 50- to 1000-fold depending on a detected physical amount, when the resistance value of a conductor through which the minor signal propagates varies, this effect is directly amplified. Thus, such a device reacts sensitively to changes in the resistance of a portion with a fine sectional structure such as a polysilicon contact that electrically connects a polysilicon resistor to a metal wire.
Earlier designs have proposed that a variation in the resistance value of the polysilicon resistor or a variation in contact resistance be suppressed. For example, Japanese Patent Application Publication No. 9-232521, incorporated herein by reference, discloses a semiconductor device in which the resistance value of the polysilicon resistor is adjusted by monitoring this value, while performing thermal treatment in order to diffuse impurities through a boro-silicate glass (BSG) film over the polysilicon resistor, as well as a manufacturing method therefor. Furthermore, Japanese Patent Application Publication No. 11-150010, incorporated herein by reference, discloses a method of adjusting the resistance value by regulating the position of the contact between the polysilicon resistor and the metal wire. To cite yet another example, Japanese Patent Application Publication No. 11-330365, incorporated herein by reference, discloses a semiconductor device in which a variation in contact resistance is restrained by forming a nitride film over the polysilicon resistor to suppress damage caused by over-etching of the polysilicon resistor when a contact hole is opened, as well as a manufacturing method therefor. All of these proposals attempt to realize, during the manufacturing stage, a resistance value as close to a design value as possible, and are not intended to suppress variations over time in resistance value in high-temperature environments.
The effects of temperature on a polysilicon resistor were examined as follows. The polysilicon resistor was electrically connected to the metal wire via a polysilicon contact, and these components were constructed using a procedure commonly used for semiconductor integrated circuits. As a result, it was determined that if a semiconductor device composed of a conventional polysilicon resistor and a conventional polysilicon contact is left in high-temperature environments, the resistance value of the polysilicon resistor varies more significantly than diffusion resistance, as shown in FIG. 9.
This is due to the fact that the resistance value of the polysilicon contact varies sharply in high-temperature environments, for example, when the temperature of the environment is higher than room temperature. FIG. 9 is a graph showing variations in the resistance value of the polysilicon resistor and the resistance value of the diffused resistor in the semiconductor device composed of the conventional polysilicon resistor when the device was left at 220° C. The maximum temperature at which semiconductor integrated circuits for automobiles or the like are operated is approximately 150° C., but acceleration tests were conducted with the temperature set at a larger value.
Accordingly, if such a semiconductor device composed of a conventional polysilicon resistor is used in an amplifying circuit used in high-temperature environments, amplifying rate varies over time in this high-temperature environment, making it difficult to maintain reliability over an extended period.
The present invention is provided to solve the above referred problems, with a semiconductor device comprising a polysilicon resistor having a resistance value that does not vary substantially even in high-temperature environments.